During the last half-decade "fast packet" multiplexing specifications and "frame switching" services such as Frame Relay (FR) have become increasingly popular as an efficient and economical means to transport large quantities of data across existing wide area digital data transmission networks (WANs). This is due, at least in part, to the fact that a frame switching approach to digital data transmissions necessarily limits each user of the network to only an amount of bandwidth actually needed for a particular given communication, thus, freeing up bandwidth for other users. In a frame switching scheme, digital data is communicated throughout the digital network in bursts or packets called "frames" which are variable in size. The Frame Relay packet data protocol now used to access many WANs evolved from the well known CCITT (now known as the International Telecommunications Union - Telecommunications Standardization Sector or ITU-TS) X.25 packet interface specification by removing certain features that are no longer needed given the many recent improvements in digital signal transmission technology. Essentially, the ITU-TS Frame Relay access interface protocol is similar to statistical multiplexing with the added condition that each frame contains information from only one user. The Frame Relay protocol as provided by ITU-TS recommendations 1.441, Q.921 and ANSI Ti.602, which are hereby incorporated by reference, provide the definition for the arrangement of digital information within a frame. Basically, each frame packet is composed of three parts: 1) an address, 2) a variable-length information field or "payload" and 3) a CRC or checksum for error checking (as illustrated, for example, in FIG. 3).
Physically, a conventional Frame Relay network is implemented by interconnecting Frame Relay access/interface "nodes" with a wide area digital data transmission network. At each Frame Relay node there are interface connections to users and/or switches to other nodes. The protocol and the arrangement of the physical network is designed so as to allow a user to reach many other users through a single connection to the network by simply changing the frame address. The Frame Relay network of itself does not terminate or process any protocol nor change or interpret the user data transported. Frame Relay is transparent to user data. A Frame Relay switch at a node either sends (relays) a particular frame over a particular network connection based on the frame address and a set of routing tables relating addresses to specific network connections or else discards the frame for any form of error. Consequently, Frame Relay is basically a "connection-oriented" protocol that establishes a logical connection for the duration of the communication and is currently implemented only as a permanent virtual circuit (PVC) service. (SVC service will probably exist in the future).
Although the Frame Relay protocol and network offers an economical alternative data transport mechanism, much of the installed data terminal equipment (DTE) of preexisting local area networks (LANs) cannot access a Frame Relay network because the DTE of the local network lacks support for the protocol. An external device called a Frame Relay Access Device (FRAD) solves this problem by functioning as an interface between a Frame Relay network and other existing data sources. Typical data source protocols supported by conventional FRADs include X.25, TCP/IP/Ethernet, SNA, Token Ring, and High Level Data Link Control (HDLC). However, as of this writing, applicants are aware of no commercially available FRAD which supports the SS7 (Signaling System No. 7) protocol.
The ITU-TS Signaling System No. 7 (SSN7 or SS7) protocol is currently implemented worldwide as the signaling system protocol for most digital telco trunk systems. Within the United States, the standard for telco network signaling is the so called SS7 protocol, as defined by ANSI standards Nos. T1.110 through T1.116, hereby incorporated herein by reference. Basically, SS7 is a general purpose common channel signaling (CCS) protocol. By definition, in a CCS signaling method a single channel conveys, by means of labeled messages, signaling information related to a multiplicity of circuits, or other information such as that used for network management. CCS can be regarded as a form of data communication that is specialized for various types of signaling and information transfers between processors in telecommunication networks. The SS7 protocol for CCS is optimized for operation over 64 kbps and 56 kbps digital channels, although it is also employed over analog channels at lower data rates (e.g.,4.8 kbps).
Telecommunications (telco) networks, as distinguished from digital data transport networks, actually comprise two logical networks. The first, conventionally referred to as a "transport" network, is a network which carries voice, data and other subscriber oriented signals between communications endpoints. The second is a digital signaling network over which various network elements communicate signaling messages to one another in order to control the operation of the transport network. Those elements include, for example, service switching points (SSPs) various databases or service control points (SC) deployed within the network and signaling transfer points (STPs), which route the signaling messages among the other elements. Thus, for example, messages transmitted over the signaling network are used to set up and tear down circuits interconnecting calling and called locations. They are also used to access and obtain information from such databases as calling card databases and databases which contain information about how to route "800" and other special service telephone calls.
Conventionally, a signaling system uses signaling links for transfer of signaling messages between SSPs and SCPs. Arrangements are provided to ensure reliable transfer of signaling information in the presence of transmission disturbances or network failures. These include error detection and correction on each signaling link. In addition, signaling systems of this sort usually must be equipped with a redundancy of signaling links, and include functions for automatic diversion of signaling traffic to alternative paths in case of link failure. Consequently, the operation of a telco signaling system with diverse facilities spread over a large geographical area or attempts to integrate separate or expand local regional signaling systems into a large WAN can be prohibitively expensive and difficult to maintain. A cost effective and low maintenance alternative mechanism for interconnecting and expanding telco signaling systems would provide a much needed solution.
Since frame switching services have grown in popularity and reliability, digital data transmission networks employing Frame Relay protocol have increased in number as well as geographic coverage, and moreover, have become a very ecomonical alternative service for transporting digital data reliably over long distances. Therefore, the present invention contemplates utilizing existing Frame Relay infrastructures to economically interconnect and expand various SS7 facilities.
Unfortunately, using a Frame Relay network to transport SS7 traffic is feasible in theory but inefficient. This is because the SS7 protocol defines user data handling and link managing procedures that continually use all available link bandwidth. For example, in an SS7 protocol communications link "signaling units" are continuously transmitted whether or not message information is actually transferred. Accordingly, running SS7 protocol over a Frame Relay network using a conventional frame relay access device (FRAD) would require a CIR (Committed Information Rate) greater than the standard data rate of an SS7 link (i.e., since SS7 protocol alone would utilize the entire bandwidth, SS7 plus FR would not be possible). Therefore, the present invention contemplates a viable approach: select only the non-redundant and essential SS7 protocol signaling units from an SS7 data stream to transport across the Frame Relay network and subsequently reconstruct the appropriate SS7 protocol data stream at a destination interface. This approach eliminates the need to occupy all the available bandwidth of the Frame Relay network and essentially provides a seamless and economical logical data link between geographically separated SS7 facilities.
In accordance with the present invention, a specially enhanced frame relay access device, the SS7 FRAD, recognizes and eliminates redundant information from an SS7 protocol data stream signal source and encapsulates essential SS7 signaling units into Frame Relay (FR) protocol packets for introduction into a Frame Relay network. The SS7 FRAD also performs the complementary interfacing functions including extracting and decapsulating SS7 data from received FR frames and the generating of a complete and continuous SS7 data stream. Basically, SS7 FRADs are used in pairs. By connecting the FR interface of one SS7 FRAD to the FR interface of a mate SS7 FRAD across a digital data network, a pair of SS7 FRADs can be made to support the operation of a single SS7 link. Moreover, the present invention further contemplates a signaling system arrangement wherein various SS7 facilities (different networks and/or signaling/transfer points) can be interconnected in an economical fashion by exchanging signaling units and other information across an existing Frame Relay network through the use of two or more enhanced FRAD interfaces in accordance with the present invention.